The recycling of printed waste paper corresponds to a rapidly developing industrial reality, and is consequently becoming a fairly widespread technique which there is therefore no need to describe in detail. In summary, the recycling consists in carrying out two essential operations, namely a first deinking operation and a second bleaching operation.
The first so-called "deinking" operation is an indispensable stage which is currently also well known. The second so-called "bleaching" operation only becomes necessary when it is desired to obtain recycled pulps of high quality, in other words with a high level of whiteness.
The deinking operation employs two main phases:
the detaching of the ink from the fibrous support, carried out in a pulper simultaneously with the suspension of the waste paper, generally with the addition of one or more chemical reagents; PA1 then, the removal of this ink, after it has been detached, from this pulpy suspension, which removal is carried out, for example, by flotation or by washing. PA1 the said paper is converted into a pulpy suspension; PA1 the ink is then removed from this suspension in two distinct and successive phases I and II, followed by a thickening stage; PA1 lastly, the pulp deinked in this way is sent into the use circuit of the pulp, is characterised: PA1 in that the first phase I, during which the waste paper is disintegrated into a pulpy suspension, the ink is then removed initially and the partially deinked pulp is thickened, takes place in a neutral or acid medium; PA1 and in that the second phase II, in which the pulp thus thickened is recovered, reagents are added and, lastly, the ink is removed for a second time and the pulp is thickened again, takes place in an alkaline medium.
Indeed, the detaching of the ink from the fibrous support is traditionally carried out in an alkaline medium. Sodium hydroxide, which is generally used, has a twofold beneficial effect. On the one hand, it causes the cellulose fibre to swell and, on the other hand, it attacks the binders of the inks to the fibrous support.
For some years, a continually growing development of newspapers printed using the flexographic printing method has been observed, in particular due to the numerous advantages which it offers as compared with the traditional printing methods.
Nevertheless, paper printed using the flexographic method has the serious disadvantage of being difficult to deink, and hence difficult to recycle, using the traditional deinking methods. Indeed, the binder for flexographic inks consists of acid acrylic resins saponified by an amine. If the paper printed in this way is treated with an alkaline medium, the binder dissolves, causing virtually the isolation of the pigment. Since the latter is very fine and, furthermore, does not have a hydrophobic nature, it does not float and offers a larger specific surface area, consequently better absorbing the incident light, and hence affecting the residual whiteness of the pulp.